1. Field of the Invention
The present invention relates to a position measuring system comprising a scale and a scanning unit movable with respect to it.
2. Discussion of Related Art
As a rule, besides incremental signals regarding the relative offset of two parts that are movable with respect to each other, known position measuring systems provide also so-called reference pulse signals. With a defined relative position of the parts that are movable with respect to each other it is possible to provide an absolute reference of the position measurement by the reference pulse signals. Reference marking fields are arranged at one or several locations of the scale for generating the reference pulse signals. Inter alia, the scanning unit of the position measuring system includes at least one scanning plate with one or several scanning graduations by means of which it is possible to generate a reference pulse signal corresponding to the relative position of the scale and the scanning unit to be detected.
A number of options on the part of the scanning unit are known regarding the arrangement of the reference marking fields on the scale, or of the scanning graduations required for scanning the same.
For example, U.S. Pat. No. 4,263,506, the entire contents of which are incorporated herein by reference, discloses the arrangement of the reference marking fields on the scale laterally adjoining the graduation track with the incremental graduation.
Besides this, it is also possible to integrate the reference marking fields at at least one reference position directly into the graduation track with the incremental graduation, such as is proposed, for example, in U.S. Pat. No. 3,985,448, the entire contents of which are incorporated herein by reference. For example, one or several ridges or lines of the incremental graduation can be omitted at the desired location of the scale.
Further variations regarding the integration of reference marking fields into the track with the incremental graduation are known from DE 35 36 466 A1 and U.S. Pat. No. 4,866,269, the entire contents of which are incorporated herein by reference. In these publications it is proposed to design the reference marking fields in the incremental graduation as a periodic sequences of lines or ridges, or to use areas with changed optical properties differing from the remaining incremental graduation as reference marking fields.
In particular in the case of high-resolution optical position measuring systems, in which the scanning signals are generated based on the interference of superimposed partial beams of rays, there is the requirement in connection with the generation of reference pulse signals to also generate high-resolution reference pulse signals.